1. Technical Field
The present invention relates to a porous polyolefin resin film which can be utilized as packing, hygiene, livestock, agricultural, building, medical materials, a separation film, a light diffusing plate, and a separator for a battery, and particularly relates to a porous polyolefin resin film which can be utilized as a separator for a nonaqueous electrolytic solution battery.
2. Background Art
A porous polypropylene film having a large number of intercommunicable micropores is utilized in various fields. For example, the porous polypropylene film is utilized as separation films to produce ultrapure water, purify chemicals, and treat water; waterproof moisture-permeable films for use in clothes and sanitary materials; and separators for use in batteries. As a method of producing the porous polypropylene film, a method of melting an inorganic filler such as calcium carbonate, barium sulfate or the like and mixing the calcium carbonate, the barium sulfate or the like with the polypropylene to form a film and thereafter stretching the film to make the film porous is known. But the porous film obtained by carrying out the above-described method has a problem that a defective dispersion portion is formed owing to a low compatibility between the inorganic filler and the polypropylene. Thereby pin holes are liable to be formed in stretching the film to make the film porous. In addition the inorganic filler drops from the resin composition while the porous polypropylene film is being produced or used and contaminates a production line.
Various methods of stretching a propylene sheet containing a β crystal are proposed. In Japanese Patent Publication No. 3443934 (patent document 1), the porous film obtained by stretching the β crystal-containing propylene sheet to which the amide compound has been added is proposed. In WO Publication 2002/066233 (patent document 2), the method of producing the porous polypropylene film by sequentially biaxially stretching polypropylene containing needle-shaped β crystal is described. As the methods of producing the porous polypropylene film using the polypropylene containing the β crystal as disclosed in the above-described publications, the stretch ratio in the longitudinal direction (hereinafter referred to as “MD stretch ratio”) and the stretch ratio in the width direction (hereinafter referred to as “TD stretch ratio”) are increased to improve the air-permeable characteristic of the porous polypropylene film by increasing the number of pores in the porous film.
In recent years, a large lithium-ion battery has come to be used for a hybrid car, tools, and the like and generate a high output. It is necessary to use a separator having a low electric resistance to allow the battery to have a high output. To this end, in the case where the porous polyolefin resin film is used as the separator for the battery, the separator is demanded to have a high porosity and a high air permeability. Because the battery has come to generate a high output, the temperature inside the battery rises. To prevent a short-circuit from occurring between positive and negative electrodes owing to a thermal contraction of the separator, the separator is demanded to have a low thermal contraction percentage at a high temperature.
To comply with the above-described demand, in Japanese Patent No. 2883726 (patent document 3), there is proposed a method of producing the porous film by processing the resin composition containing the β crystal-containing polypropylene resin to which the inorganic substance has been added (in U.S. Pat. No. 2,883,726 (patent document 3) into a sheet and stretching the sheet in a specific condition. The method proposed in Japanese Patent Application Laid-Open No. 09-176352 (patent document 4) is similar to the above-described method except that the resin particles are added to the β crystal-containing polypropylene resin.
In Japanese Patent Application Laid-Open No. 2005-171230 (patent document 5), there is proposed a method of producing the porous film by adding 0.2 to 10 parts by weight of the resin incompatible with polypropylene and having a high melting point to the β crystal-containing polypropylene resin.